1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method wherein tone correction information is defined based on the density of a formed patch image.
2. Description of the Related Art
As an image forming apparatus such as a printer, a copier and a facsimile, there has been known, for example, an apparatus adapted for optimum color-tone recording by varying the light-emission time of a semiconductor laser based on an image signal sent from a host computer or a scanner. In this apparatus, RGB image signal are converted into CMYK color-tone data for each pixel representing toner colors. Subsequently, with reference to a look-up table stored in a non-volatile memory in the apparatus, the CMYK color-tone data items are corrected for the tone levels thereof based on gamma characteristics of the image forming apparatus (a relationship between a density value indicated by the input image information and a corresponding density value of an image output). An image is formed based on the corrected color-tone data. That is, the image forming apparatus takes the following procedure to form an image of excellent tone characteristics. This procedure includes the steps of: previously storing, in a memory, a look-up table for each of the CMYK colors as tone correction information representing corrected tone levels in correspondence to input tone levels; correcting the color-tone data for each of the CMYK colors using the look-up table; and forming an image based on the corrected color-tone data.
Unfortunately, the image forming apparatus of this type is susceptible to variations in its characteristics, or particularly the gamma characteristics thereof, in association with environmental changes such as of temperature and humidity or with aging. Accordingly, a serious degradation of the tone characteristics may result. Such a phenomenon is particularly noticeable in a non-magnetic mono-component development system. As a solution to this problem, there has hitherto been proposed a technique (hereinafter, referred to as “tone correction technique”) for achieving the excellent tone characteristics by correcting the look-up table according to the characteristics variations. According to this technique, a plurality of test patch images individually having different tone levels are prepared, whereas an image density of each patch image of each tone level is detected. Then, the gamma characteristics of the image forming apparatus are determined from the detection results so that the look-up table may be corrected based on the characteristics thus determined.
In order that such a tone correction technique accomplishes more accurate correction of the look-up table, the greatest possible number of patch images of different tone levels are required for more accurate determination of the gamma characteristics of the apparatus. For more accurate detection of the density of a patch image of a tone level, on the other hand, the patch image must be formed in a sufficiently greater size than that of a detection spot of a density sensor for detecting the image density thereof. Thus, the increase in the number of tone levels to be represented by the patch images results in the increase in process time and toner consumption.
Conversely, if the patch image representing each tone level is decreased in the area thereof, the process time and the toner consumption will be decreased. This approach requires a density sensor of high resolution, which is capable of focusing on a smaller detection spot. Unfortunately, the density sensor having such a high resolution is expensive. Furthermore, the reduction of the detection spot size entails increased influences of minor image defects or noises on the detection results so that the detection accuracy is detrimentally lowered. In addition, more positive detection of the density of the small-sized patch image by means of the density sensor dictates the need for complicated control for high-precision alignment between the patch image and the density sensor. This also involves a cost increase.
In this connection, the conventional tone correction technique generally takes the following approach in order to acquire tone correction information while accomplishing the reduction of process time and toner consumption. Specifically, there are formed typical patch images representing several of the multiple tone levels (e.g., 256 tone levels), so that the several tone levels may be interpolated for estimation of the gamma characteristics. However, the more recent years have seen an increasing user demand for images of higher quality whereas the conventional technique does not necessarily afford the look-up table of sufficient accuracy. Consequently, the user demand cannot always be satisfied fully.